Conventional starting aids for high intensity discharge lamps, and in particular high pressure sodium (HPS) lamps, have traditionally consisted of a tungsten wire wrapped around, or positioned alongside, the ceramic arc tube. The tungsten wire is welded to a frame member or an electrode feedthrough during the manufacturing process to provide electrical contact with the lamp""s power supply. This basic type of starting aid has been manufactured for many years and generally performs reliably over the life of the lamp. However, pure tungsten wire is relatively expensive and labor and time are required to form the welds.
More recently, lamp manufacturers have used an integral starting aid which consists of a printed stripe directly sintered to the ceramic arc tube. The stripe is made of either pure tungsten or a tungsten/alumina cermet. For example, U.S. Pat. No. 5,541,480, which is incorporated herein by reference, describes a polycrystalline alumina (PCA) arc tube having an integral tungsten ignition aid which is applied as a tungsten-containing paste prior to sintering the arc tube to translucency. These integral starting aids reduce the costs of manufacturing but may not perform as reliably as tungsten wire starting aids. In particular, because the starting aid is applied as a thin stripe directly to the surface, any mismatch in the thermal expansion coefficients of the ceramic arc tube and starting aid materials becomes important. The high temperature of the operating arc tube coupled with the on-off thermal cycling which occurs throughout the operating life of the lamp can cause the thin stripe to break and lose electrical continuity thereby rendering it inoperable. In addition, the geometry of the starting aid is also limited by thermal expansion mismatches. If the starting aid is applied too thickly, the induced stress from the thermal expansion mismatch can cause the arc tube to crack. Therefore, it would be desirable for the material comprising the starting aid to have a thermal expansion coefficient which closely matches that of the arc tube material.
It is an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to provide an integral starting aid for high intensity discharge lamps wherein the thermal expansion coefficient of the starting aid material is closely matched to that of the ceramic arc tube.
It is still another object of the invention to provide an integral starting aid which is capable of withstanding the high temperatures and thermal cycling of the operating arc tubes of high intensity discharge lamps.
In accordance with one object of the invention, there is provided an integral starting aid comprised of a conductive, refractory metal nitride stripe applied directly to the surface of a ceramic arc tube. The metal nitride stripe may be applied by a number of conventional means including aerosol spraying, ink pen, ink-jet, or vapor deposition. In one aspect, the integral starting aid is comprised of a thin stripe containing titanium nitride or zirconium nitride. The starting aid is sintered with the ceramic arc tube to bond it to the arc tube surface.